I wonder what impact a cantilevered fretboard would have to the design of the upper transverse brace in a classical guitar. You would pretty much remove any need for the UTB to support against neck and fretboard rotation, I would think. You would still need to support that wide part of the upper bout, but could you use a lighter, more delicate brace? What about additional support to the neck block to replace the support lost from floating the fretboard? With the low loads from classical strings, how much mechanical support is really required? Part of my thinking is that coming from archtops to contemplating and planning a classical build, I want to explore a raised fretboard (all my player friends tell me they much prefer a raised fretboard), so the next step to a floating fretboard seems very natural. I must say, all the reading and learning I am doing about very traditional classical guitar construction seems very un-natural to me. I build the neck as the very last step - they build the neck as the very first step, attach the top to it, and build the guitar around that! Seems very odd to me!

The Spanish method is to build the guitar around the neck on a solera. Not everybody does it that way, nor, indeed, have the Spaniards always done so. It's actually a fairly recent 'tradition', as I understand it. Many modern makers use a plug in neck and there is not a lot of resistance to it in the Classical guitar world that I know of. For what you want to do, I'd plug in the neck for sure.

All of the rotational force will still be there, whether the fingerboard touches the top or not. Without an arch to stiffen the upper bout you need to put in a structural brace of some sort. I've been using 'A' braces on flat top and Classical guitars for a long time, and they help a lot. On a Classical the UTB is pretty small. The 'A' braces are about 1/4" square, and run through the UTB at full height, along side a trapezoidal soundhole doubler, and are tapered at the bottom end to partially inlet into the waist brace. Since the lower ends of the 'A' braces are only the height of the doubler at the lower end (about 2mm) they don't require a lot of support. I cut them down to a triangular shape at the end that goes halfway into the waist brace, which is notched for them. If you are careful the fact that they're been inlet won't show. They're inlet into the upper block at full height at the top.

I've found this to work very well in resisting neck displacement. The 'A' brace takes the pressure as a compression load, and distributes it across the top. This beats the usual setup, where the load is taken by glue lines in shear. I've had a couple of guitars with this feature dropped in such a way as to hit the back of the neck, leaving a deep ding, with no problems. One was a 12-string....

Maybe take a look at a some images of Miguel Rodriguez soundboards to get an idea of bracing above the waist. Kind of an A brace like Alan mentioned, but the "straps" of the Rodriguez are a bit wider and serve double duty as soundhole reinforcement. They run thru the UTB and I believe are scarfed into the heel. Haven't tried it yet but I like the idea so far.